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The development of air-conditioners using CO2 refrigerants in place of freon-based refrigerants is underway. In these air-conditioners, the temperature of the heat-exchange cycle increases to 453 K [1]. The intergranular corrosion susceptibility of the Al-Mn-Cu alloy was investigated using the anode galvanic dissolution test after performing brazing and heat treatment at 453 K. The intergranular corrosion susceptibility was dependent on the amounts of Mn and Cu, and heat treatment at 453 K resulted in an increase in the intergranular corrosion susceptibility. In the case where brazing was not followed by heat treatment at 453 K, TEM observation revealed Al-Cu intermetallic compounds with Al-Mn precipitates on the grain boundary. On the other hand, when heat treatment was carried out at 453 K, the growth of both Al-Cu intermetallic compounds and their precipitates was observed on the grain boundary with no Al-Mn precipitates.